Sponsored by Democratic Sen. Robert F. Wagner of New York, the Wagner Act established the federal government as the regulator and ultimate arbiter of labour relations. It set up a permanent three-member (later five-member) National Labor Relations Board (NLRB) with the power to hear and resolve labour disputes through quasi-judicial proceedings. Specifically, the NLRB was empowered to decide, when petitioned by employees, if an appropriate bargaining unit of employees existed for collective bargaining; to conduct secret-ballot elections in which the employees in a business or industry could decide whether to be represented by labour unions; and to prevent or correct unfair labour practices by employers (later also by unions). The act prohibited employers from engaging in such unfair labour practices as setting up a company union (such as Rockefeller's Employee Representation Plan) and firing or otherwise discriminating against workers who organized or joined unions. The act also barred employers from refusing to bargain with any such union that had been certified by the NLRB as being the choice of a majority of employees. Fiercely opposed by Republicans and big business, the Wagner Act was challenged in court as a violation of the “freedom of contract” of employers and employees and as an unconstitutional intrusion by the federal government in industries that were not directly engaged in interstate commerce, which Congress was empowered to regulate under the commerce clause (Article I, section 8). The U.S. Supreme Court eventually upheld (5–4) the constitutionality of the Wagner Act in National Labor Relations Board v. Jones & Laughlin Steel Corp. (1937).
The Wagner Act was significantly weakened by the Taft-Hartley Act of 1947, passed by a Republican-controlled Congress over the veto of Democratic Pres. Harry S. Truman. The Taft-Hartley Act prohibited the closed shop (an arrangement that makes union membership a condition of employment), allowed states to prohibit the agency shop (an arrangement that requires employees who are not union members to pay fees to a union to cover the costs of its bargaining on their behalf), narrowed the definition of unfair labour practices, and specified unfair union practices, among other provisions. Following adoption of the Taft-Hartley Act, a number of states enacted so-called “right to work” laws, which banned both closed and agency shops. The Wagner Act was further amended by the Landrum-Griffin Act (1959), which banned secondary boycotts and limited the right to picket.
The majority of this information was taken from Encyclopedia Britannica, and can be found here.